The present invention relates to a method and apparatus for controlling the operation of a relay. More specifically, the present invention relates to a method and apparatus for switching a relay ON and OFF at or near the zero crossing point of an AC waveform.
Relays are used to switch many electrical appliances ON and OFF connecting and disconnecting the appliance from an AC power source, respectively, in response to a control signal. One category of electrical appliances that are commonly controlled by relays are electronic ballasts that are switched ON and OFF in response to a sensing device such as a motion detector.
When switching an electrical appliance such as an electronic ballast or an incandescent light ON and OFF with a relay, relay contact must be made at or near specific points along the AC voltage waveform--the zero crossing points. If contact is made or broken at random points along the AC voltage waveform, contact may be made when the waveform is at a sufficient voltage level to cause high temperature electrical arcs to form between the relay contacts. The formation of these arcs is referred to as arcing. Arcing causes contact erosion and reduces the useful service life of the relay.
A consideration that must be taken into account in designing a circuit that switches relays ON and OFF near the zero crossing point is the turn-on and turn-off time of the relay, also referred to as the "make" and "break" times of the relay, respectively. FIG. 1 is a timing diagram showing the delay that occurs in making and breaking relay contact. In FIG. 1, an ON/OFF control signal 100, an AC voltage waveform 105, and a relay contact signal 110 are shown. When ON/OFF control signal 100 is switched to ON at point 115, the delay in making relay contact at point 120 is shown by time T.sub.ON. Since the voltage of waveform 105 is not near zero volts at point 120, making relay contact at this point can damage the relay.
Similarly, when ON/OFF control signal 100 is switched to OFF at point 125, the delay in breaking relay contact at point 130 is shown by time T.sub.OFF. Since the voltage of waveform 105 is not near zero volts at point 130, breaking relay contact at this point can damage the relay. Many relays have make and break times that are equal to each other (T.sub.ON =T.sub.OFF). Other relays, however, have differing make and break times.
Previous circuits have been designed to account for the turn-on time of a relay and switch the relay on near the zero crossing point. One example of such a circuit is in U.S. Pat. No. 4,642,481 issued to Bielinski, et al. Another example is in U.S. Pat. No. 5,267,120 issued to Graff, et al. These prior art circuits, however, are complicated in both structure and number of components making them expensive to use on a mass production scale in highly competitive markets. Additionally, none of the prior art relay control circuits provide different delay times to ensure that relays which have different turn-on and turn-off times are both opened and closed at the zero crossing circuit.